1. Field of the Invention
This invention relates to a diffusion plate, and more particularly to a diffusion plate applicable to the focusing screen of a single lens reflex camera or a cinecamera.
2. Description of the Prior Art
Frosted glass has been used for the focusing screen of a camera. However, the focusing screen of frosted glass has suffered from difficulties in brightness. Thus, applicant has proposed in U.S. application Ser. No. 105,884, filed Dec. 20, 1979, now U.S. Pat. No. 4,336,978 a method of making a diffusion plate having microlens-like curved surfaces arranged periodically in two dimensions on the surface of the substrate thereof, and further having curved surfaces formed between the microlens-like curved surfaces.
When the diffusion plate made by this manufacturing method has been incorporated into a camera, it has been found that even if the aperture is stopped down, the marginal portion thereof does not become dark and the diffusion plate provides a bright focusing screen in which no speckle is seen as in frosted glass. However, this focusing screen basically has a diffraction grating structure (periodical structure) and therefore, (1) when there is a periodical structure in an object to be photographed, and (2) when the pitch (usually 30-50.mu.) of the concentric circles of a Fresnel lens lying on the reflection side surface of the focusing screen is similar to the pitch of the diffraction grating structure of the focusing screen, there are created moire stripes which make the view-finder image difficult to view.
The diffusion plate of the present invention alleviates the influence of the periodical structure of the microlens by minute concavo-covexity irregularly arranged on the microlens-like curved surfaces.
Focusing screens having minute concavo-convexity provided on the microlens-like curved surfaces are already commercially available. However, no curved surfaces are formed between the microlenses of such a focusing screen.